Control system for synchronous machines



May 12, 1942. s. c. EWING CONTROL SYSTEM FOR SYNCHRONOUS MACHINES Fil ed Aug. 2'7, 194,0

Na i.

Inventor Samuel C. Ewing,

by His Attorney.

claims.

Patented May 1942 T OFFICE ooN'rnoL SYSTEM ron SYNCHRONOUS MACHINES I Samuel C. Ewing, Scotia, N. Y.,

Company,

eral Electric York assignor to Gena corporation of New Application August 27, 1940, Serial No. 354,374

Claims. (01.172-289) completes for its closing coil 6 'a locking cir- My invention relates to control systems for synchronous machines and particularly to synchronous motor control systems, and an object of my invention is to provide an improved arrangement for controlling the application and removal of the field excitation of a synchronous machine. i

In United States Letters Patent 1,896,074 granted February 7, 1933 on an application filed by me and assigned to the assignee of this applic ation there is disclosed and claimed a synchronous motor field excitation control system, and my present invention is an improvement of that arrangement in that the improved arrangement requires fewer timing devices and, therefore, is a cheaper and simpler arrangement.

My invention will be better understood from J the following description when taken in connection with the accompanying drawing the single figure of which is a diagrammatic showing of asynchronous motor control system embodying my invention, and the scope of my invention will be pointed out in the appended Referring to the drawing, I represents a synchronous motor having an armature winding 2 and a field winding 3. 4

In order to simplify the disclosure, I have shown my invention in connection with a full voltage starting arrangement for the -synchro-- motor armature winding to start the motor from rest and bring it up to approximately synchronous speed as an induction motor. It is to be understood, however, that my invention is applicable to any other suitable arrangement for starting and accelerating'the synchronous motor to approximatelysynchronous speed as an induction'motor, As shown in the drawing, the line switch t has a closing coil 6 which is arranged to be connected across one phase of the supply circuitii when a suitable start switch 7 is closed. flhisv start switch 1 may be, a float switch, timeswitch, thermostat, or any other suitable control device, examples of which are well known in theart, for initiating the starting operation of the motor. In the circuit of contacts of a suitable manually controlled stop switch 8 which is operated when it is desired to stop-the'moton', The line switch 4, when closed,

field switch I3 is charge resistor i cuit which is independent of the start switch 1 so that this switch may be opened without effecting the opening of the switch I.

The field winding 3 of themotor I is normally short-circuited through a discharge resistor II and is arranged to be connected to a suitable source of excitation i2 when the motor I reaches a predetermined speed. In the particular arrangement shown in the drawing a two-position provided for controlling the field winding 3. In one of its switch t3 connects the disacross the terminals of the I and in its other position it con-. nects the field winding 3 to the source of excitation l2. The field switch I3 is provided with a closing coil M which, when energized, operates the switch so as to disconnect the field winding 3 from the discharge resistor H and connect the field winding 3 to the source of excitation l2. For effecting the energization of the closing coil l4 when the motor l reaches substantially synchronous speed, I employ a time relay l5 having a winding l6 connected in series with a half Wave rectifier i1 across a portion or all of the dis- I. With such an arrangement the induced current which flows through the motor field winding 3, while the motor 8 is operating below synchronous speed, causes sufficient pulsating current to flow through the windingji 6 to maintain the armature of the relay 15 in its attracted position to close i9 and'open i8 until the motor reaches approximately synchronous speed at which time the frequency of the induced current becomes so low that the portion of each'cycle of induced field current during which no current flows through the relay connections of the positions the field charge resistor H field winding 3,

the closing coil 6 I provide thenormally closed p sition,

armature to he restored to its normal position,

In the arrangement shown, the relay i5 is provided with contacts it which are closed when the relay armature is in its attracted position and with contacts it which are closed when the relay armature is in its normal position. The contacts [8 are connected in series with the closing coil M of the field switch 53 so that it can he closed only when the frequency of the induced field current is below a predetermined value and, therefore, the motor speed is above a predetermined subsynchronous speed.

The contacts I 9 of relay it are connected in the energizing circuit of a suitable time delay drop-out relay 2i which, when in its energized closes its contacts 22, which are also in series with the closing coil I4 of the field switch I3. Therefore, in order to close the field switch I3, the relay I5 must first close its contacts I9 so as to effect the energization of relay 2| and then must close its contacts I8 while the relay 2| is still in its energized position. When the field switch I3 closes, its auxiliary contacts 24 complete a shunt circuit around the contacts I8 of the relay I5, and its auxiliary contacts 25 in series with the contacts 26 of a suitable pullout relay 21 complete a shunt circuit around the contacts 22 of the relay 2|. The pull-out relay 21 is connected to the motor I in any suitable manner, examples of which are well known in the art, so that its contacts 28 are opened when the motor I falls out of synchronism. In the particular embodiment shown in the drawing, the pull-out relay 21 is a power-factor relay which is connected to the armature circuit of the motor I and which is arranged in any suitable manner so that it opens its contacts 25 in response to the power-factor of the motor armature circuit decreasing below a predetermined lagging value.

Since the relay 2| is a time delay drop-out relay, the contacts 22 thereof remain closed for a suflicient length of time after the field switch I3 closes to prevent any momentary opening of the contacts 26 of the pull-out relay 21, during the synchronizing operation of the motor I, from effecting the opening of the field switch I3.

In some cases it has been found in practice that, if the motor load is removed at the time of pull-out and field removal, as for example in an arrangement in which the opening of the field. is arranged to effect the operation of an unloader, the speed of the motor may not drop to a sufliciently low value after the opening of the field'switch to cause sufiicient current, to fiow through the winding I6 of'the relay I5 to pick upthis relay. In order to insure that the relay I5 moves to its energized position and effects the energization'of relay 2| under such conditions, I provide the relay I5 with a second winding 30 the circuit of which is completed across one phase of the supply circuit 5 through a suitable rectifier 3| when the line switch 4 is closed, the field switch I3 is opened, and the relay 2| is in its deenergized position. Therefore, when the motor I is pulled out of step, there is no possibility of the motor running indefinitely without field-excitation due to the speed of the motor remaining too high after pull out to effect the proper operation of the relays I5 and 2|.

The operation of the embodiment of my invention shown in the drawing is as follows:

When it is desired to start the motor I, the start" switch I is closed so that an energizing circuit is completed for the closing coil 6 of the line switch 4 across one phase of the supply circuit 5. This energizing circuit also includes the normally closed contacts of the stop switch 8 and the contacts 31 of the field switch I3. The energization of the closing coil 5 closes the line switch 4 so that normal voltage is applied to the motor armature winding 2 to start the motor I from rest and accelerate it as an induction motor to approximately synchronous speed. By closing its auxiliary contacts 33, the line switch 4 completes for the closing coil 6 a locking circuit, which is independent of the start" switch I, so that this switch may be opened without effecting the opening of the line switch 4. By closing its auxiliary contacts 34, the line switch 4 completesacross one phase of the supply cir- 75 the winding I6 are such that the relay maintain closing coil I4 so that the field switch I3 isiopened c'uit an energizing circuit for the winding 30 o the relay I5 so that this relay is immediatel moved to its, operated position. This energizin circuit also includes the rectifier 3|, the contact 35 of the relay 2|, and the auxiliary contacts 3 of'the open field switch I3. As soon as the motor armature winding 2 i energized, a voltage of slip frequency is induce in the motor field winding 3, and this voltag causes a current of slip frequency to flow throug the field winding 3 and the discharge resistor II and causes a pulsating current to fiow throug the winding I6 of the relay I5. Until the moto I reaches substantially synchronous speed, th magnitude and frequency of the current throug its armature in its attracted position so tha the contacts I 9 are closed, and the contacts I are open. The closing of the contacts I9 of rela I5 completes an energizing circuit for the wind ing of relay 2| through the rectifier 3|, contact 35 of field switch I3, and the contacts 34 of th line switch 4. By closing its contacts 20, th relay 2| completes a shunt circuit around con tacts 31 of the field switch I3 and by openin its contacts 35, the relay 2| interrupts the cir cuit of the winding 38 of relay I5. The windin I8, however, maintains the relay I5 in its energized position until the motor reaches a predetermined subsynchronous speed when the frequency of the induced current in the field wind-- ing 3 and resistor II becomes so low that the portion of each cycle during which no curren flows through the winding I6 is suificiently long to allow the relay I5 to open its contacts I8 and close its contacts I8. The closing of the contacts I8 completes, through the contacts 22 of relay 2| and the contacts 34 of the line switch 4, an energizing circuit for the closing coil I4 so that the field switch I3 is closed to connec the field winding 3 across the source of excitation I2. 'Ihe'motor then pulls into synchronism and thereafter operates at synchronous speed until the load is increased to a sufiicient value to pull the-machine out of its synchronism or thefstop switch 8 is opened to stop the motor. By opening its contacts III, the relay |5 interrupts the energizing circuit of the time relay 2|, but the contacts 22 of this relay remain closed for a predetermined time after the relay is deenergized.

By closing its auxiliary contacts 24, the field switch I3 completes a shunt circuit around contacts I8 of relay I5 and by closing its contacts 25, which are in series with the contacts 26 of the pull-out relay 21, the field switch I3 completes a shunt circuit around the contacts 22 of the relay 2| In case the motor is pulled out of synchronism for any reason, the power-factor of the motor armature circuit decreases to a sufficiently low lagging value to cause the pull-out relay 21 to open its contacts 25 in the holding circuit of the to disconnect the field winding 3 from the source of excitation I2 and to reconnect the discharge resistor I I across the field winding 3. As soon as the field switch I3 opens, the heretofore described circuit for the winding 30 of the relay I5 is completed to cause the relay I5 to be moved to its energized position and thereby effect, in the manner heretofore described, the energizetion of the relay 2|. As long as the motor continues to rotate at a sub-synchronous speed below the dropout value of the relay I5, the field switch l3 remains .an operated synchronously,

and scope of my invention.

What I claim as new and desire to secure by Letters Patent 01' the United States is:

1. In combination, a synchronous machine having an armature winding and a field winding,

means for supplying alternating current to said armature winding, a source of excitation, a field switch for connecting said source to said field winding, a relay havinga normal position and position and a winding energized in a second relay having a normal position and an operated position, means responsive to the movement of said first mentioned relay to its operated position for moving said second relay to its operated position, means controlled by said relays for closing said when said first and means for when said field switch is open and relay is in its normal position.

2. In combination, a synchronous machine having an armature winding and a field winding, means for supplying alternating current to said armature winding, a source of excitation, a

said second field switch for connecting said source to said field winding, a relay having a normal position and an operatedposition, a second relay having a normal position and an operated position, means responsive to the movement of said first mentioned relay to its operated position for moving its operated position, means for causing said first mentioned relay to move to its operated position when said motor is operating below a predetermined subsynchronous speed and to its normal position when said motor is operating above said predetermined subsynchronous speed, means for moving said first mentioned relay controlled by closing said field switch when said first mentioned relay is in its normal position and said second relay is in its operated position. v

3. In combination, a synchronous machine having an armature winding and afield winding, means for supplying alternating current to said armature winding, a source of excitation, a field switch for connecting said source to said'field winding, a relay having a normal position and an operated position and a winding energized in response to the induced current in said field machine is operating suba time delay drop-out relay, means for-energizing said time relay while said relay is in its operated position, means controlled by said relays for closing said field switch when said first mentioned relay is position, means for-opening said and means for moving said first its drop-out position.

4. In combination, a synchronous machine having an armature winding and a field windmcans for supplying alternating current to said armature winding, a source of excitation, a field switch for connecting said source to said field winding, a relay having a normal position and an operated position and a winding enersubsynchronously, a time delay drop-out relay, means for energizing said time relay while said first mentioned relay is in its operated position,

' means controlled by said relays for closing said field switch when said first mentioned relay is in its normal position and said time relay is in its energized position, means for opening said field switch, a second winding for said first mentioned relay, and means for energizing said second winding when said field switch is open and said time relay is in its drop-out position.

5. In combination, a synchronous machine having an armature winding and a field winding, an alternating current circuit connected to said armature winding, a source of excitation, a field switch for connecting said source to said field winding, a relay having a normal position and an operated position and a winding energized in response to the induced current in said field synchronously, a time delay drop-out relay, means for energizing said time relay while said first. mentioned relay is in its operated position, means controlled by said relays for closing said field switch when said first mentioned relay is in its normal position and said time relay is in its energized position, means for opening said field switch, a second winding for said first mentioned relay, rectifying means, and means controlled by said field switch and said time relay for connecting said second winding and said rectifying means in series across said alternating current circuit when said field switch is open and said time relay is in its drop-out position.

SAMUEL c. EWING. 

